Orconectes rusticus, the rusty crayfish, is an aquatic invasive spread by anglers who use them as bait. It is native to the portions of Ohio, Indiana, Illinois, and Kentucky and has spread to surrounding areas as well as northeastern United States. O. rusticus is an aggressive and rapidly spreading crayfish that displaces cogeners, reduces macrophyte and invertebrate abundance, preys on native snails, and reduces sport fish populations.

DescriptionOrconectes rusticus is a relatively large crayfish that may reach 10 cm in length and has robust claws and dark, rusty spots on either side of its carapace (Gunderson, 2008; USGS, 2010). The rusty spots are usually prominent and appear on each side of the posterolateral margins of the carapace but are reported to be not always present or well developed on rusty crayfish from some waters. A rust-colored band also appears dorsally down the center of the abdomen. Overall O. rusticus has a aquamarine, greenish color that is most pronounced on the walking legs (Wetzel et al, 2004; Gunderson, 2008). Its claws are grayish-green to reddish-brown with black bands at the tips and fairly smooth (Gunderson, 2008; ANSIS, 2007). Form I males are characterized by large claws, a hook on one pair of their legs, and hardened gonapods. The hook and the larger claws are used for grasping females during mating. Males are usually larger than females of the same age (Gunderson, 2008).

Habitat descriptionOrconectes rusticus inhabits permenant lotic and lentic environments of lakes, ponds, and streams that provide suitable water quality year-round (Gunderson, 2008). Suitable substrates include clay, silt, sand, gravel, or rock, but O. rusticus is almost always found in areas with cobble and carbonate substrates and occasionally found in habitats with gravel substrate and woody debris (Kershner & Lodge, 1995; Flynn & Hobbs, 1984; Taylor & Redmer, 1996; Gunderson, 2008). O. rusticus prefers well oxygenated water and a temperature range of 20-25°C but can withstand seasonal water temperatures of 0-39°C within its native range. In temperatures over 30°C, adults have been observed digging burrows to escape the heat (ANSIS, 2007). Systems with cobble abundance may be more susceptible to invasion. O. rusticus is most often found in depths of less than 1 m but has been collected to a depth of 14.6 m in Lake Michigan (Taylor & Redmer, 1996).

General impactsOrconectes rusticus has a range of ecological impacts on introduced environments that include competition and displacement of native crayfish, increased predation on snails, native and threatened bivalves, reduction of macrophyte abundance, reduction of sport-fish abundance, reduction of macroinvertebrate abundance, increases in periphyton activity, and other cascading trophic interactions. The wide range of impacts associated with O. rusticus, its aggressive nature, rapid expansion rates, dense populations, and ability to spread through bait trade make it a very problematic aquatic invasive.

Rusty crayfish prey on threatened, native bivalves in northeastern United States. Although native crayfish also prey on these bivalves, O. rusticus can live at very high densities so the threat of increased predator populations can harm already threatened unionid populations (Klocker & Strayer, 2004; Kuhlmann & Hazelton, 2007). O. rusticus also preys on snails and in Trout Lake, Wisconsin snails declined from >10 000 to <5 snails•m2 in one of the initially invaded areas (Wilson et al, 2004). Relative to control treatments, rusty crayfish were found to reduce the biomass of northeastern US native Lymnaea and Physa snails by >90% (Johnson et al, 2009). Furthermore, O. rusticus has been found to co-occur with Bellamya chinensis, an invasive snail with a thick shell that prevents predation by O. rusticus, in northern temperate lakes throughout the United States. The predation pressure of O. rusticus on native snail communities combined with competition and displacement by the B. chinensis has resulted in the reduction of native snail biomass (Johnson et al, 2009).

The reduction of macrophyte abundance is another important impact of O. rusticus. Small-scale, comparative, and multi-lake studies confirm that macrophyte species richness and abundance decline significantly in lakes invaded by O. rusticus (Alexander et al, 2008; Rosenthal et al, 2006; Roth et al, 2007; Wilson et al, 2004). In northern Wisconsin, studies found the proportion of sites with no macrohpyte cover to increase from 40-73% (Roth et al, 2007), and submerged macrophyte species richness to decline by as much as 80% with the invasion of O. rusticus (Wilson et al, 2004).

O. rusticus introduction is also believed to reduced sport fish populations especially pan-fish Lepomis macrochirus and L. gibbossus by either egg predation or competition with juveniles. Researchers have calculated fisheries damages of O. rusticus in Vilas County, Wisconsin to be about 1.5 million annually (Keller et al, 2008).

UsesOrconectes rusticus is a commonly used live fishing bait. The majority of its introductions are believed to be the result of their intentional or unintentional release as such (Olden et al, 2009; Perry et al, 2001; Peters & Lodge, 2009). O. rusticus preys on a reduces populations of the invasive zebra mussel Dreissena polymorpha (Perry et al, 1997; Perry et al, 2000).

Geographical rangeNative range: Indiana, Ohio, Kentucky, and Michigan in the United States. Known introduced range: Has invaded many areas surrounding it's native range. It has moved as far west as North and South Dakota, north as Canada and Maine, and south as Tennessee.

Introduction pathways to new locationsBiological control:Orconectes rusticus have reportedly been intentionally released in some locations in efforts to control nuisance weeds (Olden et al, 2009).Contaminated bait:Orconectes rusticus is a commonly used live fishing bait. The majority of its introductions are believed to be the result of their intentional or unintentional release as such (Olden et al, 2009; Perry et al, 2001; Peters & Lodge, 2009).Other:Orconectes rusticus is used as a laboratory species for school science programs. Local and national biological supply companies are known to ship live O. rusticus to schools for study (Olden et al, 2009).Pet/aquarium trade:Orconectes rusticus is transported and traded as a pet (Peters & Lodge, 2009).

Local dispersal methodsNatural dispersal (local):Water currents:

Management informationPreventative measures: Educating anglers, crayfish trappers, bait dealers, teachers, and the general public about the ecological threats posed by Orconectes rusticus will help reduce the risk of its spread to new areas (Olden et al, 2006). Monitoring boat docks, fishing areas, or setting up check points in order to halt the use of O. rusticus as bait may also be effective in preventing their establishment (Keller et al, 2008). Regulations in both Minnesota and Wisconsin now make it illegal to introduce O. rusticus into any waters. In Minnesota, it is illegal to sell live crayfish as bait and a Department of Natural Resources permit is required to commercially harvest or culture crayfish (Gunderson, 2008). Regulations regarding O. rusticus in other states differ depending on state and vector. Many states have regulations that specifically targeted the invasive rusty crayfi sh. However, these regulations were enacted reactively only after rusty crayfi sh had become established in the state. The lack of regulatory consistency among the Great Lakes jurisdictions is creating a multiple weak links problem and making success unlikely in efforts to slow the spread of O. rusticus and other invasive species throughout the region (Peters & Lodge, 2009). In Wisconsin, it is illegal for anglers to possess any crayfish. In Pennsylvania it is illegal for anglers to possess, aquarists to raise, and bait dealers or pet traders to sell O. rusticus. In Ohio, it is illegal to move crayfish from a natal lake in or for aquarists to rear O. rusticus. In Michigan, it is illegal for bait dealers and pet traders to sell and for aquarists to rear O. rusticus. In Illinois, it is illegal for anglers to possess, bait shops to sell, and for aquarists to rear O. rusticus. In Ontario, it is illegal move crayfish from a natal lake and for bait dealers to sell crayfish (Peters & Lodge, 2009)

Physical: Intensive harvest will not eradicate or control crayfish, but may help reduce adult populations and minimize some impacts.

Some researchers have suggested that nuisance populations of rusty crayfish are the result of poor fishery management and that by restoring a healthy population of bass and sunfish, O. rusticus would be less disruptive in some lakes.

Populations of Orconectes rusticus may be reduced by trapping or fish predation. Although neither practice may provide eradication both have been found to be effective means of reducing negative impacts and decreasing population sizes of O. rusticus (Hein et al, 2006; Hein et al, 2007). The use of electric fences along with hand removal in experimental plots was also found to reduce densities of O. rusticus and may have implications for macrophyte restoration efforts (Peters et al, 2008).

The control of a rusty crayfish population in Sparkling Lake, an isolated lake in northern Wisconsin by trapping adult crayfish and restricting fishing, thereby increasing fish populations and predation on small crayfish was found to effectively reduce O. rusticus populations there. To protect and enhance populations of rusty crayfish predators, the Wisconsin Department of Natural Resources instated strict regulations on smallmouth bass. Also, wire minnow traps with an enlarged (3.5 cm diameter) opening were baited with 4 to 5 frozen smelt (8–13 g each), set 1–2 m deep at ~10 m intervals, and used to capture O. rusticus. Over a 3 year period, traps and predatory fishes removed substantial portions of the rusty crayfish population. Because more crayfish were vulnerable to and removed by fish predation than by trapping, fish predation caused a larger decline in the population growth rate. However, trapping removed crayfish with the highest reproductive value and caused the largest decline in population growth rate per individual crayfish removed. Researchers estimated that traps and fish removed a total of 1,212,148 individuals and 1212 kg of crayfish over three years of removal. Together they removed approximately 55% of the population in 2003 (Hein et al, 2006). Removal trapping catch rates declined by 95% over the last 4 years of removal from Sparkling Lake (Hein et al, 2007). Trapping was found to be most effective on cobble substrates (Hein et al, 2006). A similar trapping study of O. rusticus found that captured individuals left in traps excluded uncaptured individuals from entering traps (Ogle & Kret, 2008).

The experimental use of electric fencing along with hand removal were able to significantly reduce O. rusticus densities in electric plots compared to non-electric control plots in Lake Ottawa, located in the Ottawa National Forest, Michigan. Macrohpytes Potamogeton richardsonii and Elodea canadensis were eliminated within a matter of days in the control plots and within 3 wk in the electric plots (Peters et al, 2008).

Chemical: There are means of chemical control for Orconectes rusticus. However, none currently registered have been found to selectively kill O. rusticus without effecting other species of crayfish (Gunderson, 2008). An evaluation of several potential chemical controls found a synthetic pyrethroid (Baythroid) at 25 .mu.g/L was most effective and produced a complete kill of crayfish in the pond and was also the most selective for crayfish in laboratory tests (Bills & Marking, 1992). High, sub-lethal concentrations of metolachlor (80 ppb) may interfere with the ability of O. rusticus to receive or respond to social signals and thus affect certain agonistic behavior, implications may be useful to its management (Cook & Moore, 2008).

ReproductionOrconectes rusticus reproduces sexually and oviparously. Mating typically occurs in the fall and eggs are laid in the spring, but mating is reported to also occur in late summer, early spring (USGS, 2010; Gunderson et al, 2008). Males transfer sperm to the female who stores the sperm until the eggs are laid, which usually occurs in the spring when water temperatures rise. The eggs are externally fertilized by the female and attached to the swimmerets on the underside of the female's abdonmen. O. rusticus females lay from 80 to 575 eggs (Gunderson, 2008). Since females store sperm, only one female carrying sperm is necessary to begin a new population (Gunderson, 2008; USGS, 2010).

Lifecycle stagesThe eggs of Orconectes rusticus hatch within three to six weeks depending on water temperature. Once hatched, young crayfish cling to the female's swimmerets for three to four molts. Young crayfish may stay with the female for several weeks. Juveniles undergo eight to ten molts before they mature, which may occur during the first year, but more often take two. Orconectes rusticus reaches maturity at a total length of 3.5 cm and reach a maximum length of about 10 cm. A typical rusty crayfish lives three to four years. A mature adult male molts twice a year and a female molts once hence why males of the same age are usually larger (Gunderson et al, 2008).

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